The bacterial membrane is essential to sequester the interior contents of the cell away from the external environment. Although this compartmentalization function is essential, it poses a number of challenges. Principal among these are how to organize membrane proteins in the lipid bilayer and how to remodel the membrane during growth and morphogenesis. The developmental process of spore formation in B. subtilis has served as a powerful model to address fundamental questions in membrane protein localization and membrane remodeling. We will exploit this model to ask a series of questions about how proteins and lipids work together to generate a dynamic structure (the membrane) that can grow, change in shape, and produce new compartments. The first question is that of localization. What are the spatial cues that anchor membrane proteins at particular subcellar sites? A protein called SpoIIQ anchors much of the machinery involved in the early stages of spore formation, but so far, little is known about how this central organizer is itself anchored. The second question is that of membrane movement. During the morphological process of engulfment, the mother cell membranes must be moved through the peptidoglycan (PG) layer around the forespore; the mechanism for this remains unknown. The third question is biophysical. How do membranes fuse? In the last step of engulfment, the mother cell membranes undergo fusion, generating a cell within a cell. Virtually nothing is known about how bacteria catalyze membrane fusion despite the fact that in every cell division the invaginating septal membranes fuse (resulting in binary fission). Our identification of a new candidate fusion protein (FusB) provides the opportunity to define fusion in molecular terms and to investigate how membrane fusion triggers developmental gene expression. Finally, a fundamental process underlying these and many other important cellular events is the dynamic movement of membrane proteins. We have discovered two membrane proteins that undergo directed movement in the lipid bilayer; how this directionality is achieved is completely unknown. The goal of this proposal is to elucidate the underlying mechanisms that govern the positioning and dynamics of membrane proteins and the role of membrane proteins in remodeling the lipid bilayer. Specifically we will: 1) Identify and characterize the spatial cues that anchor SpoIIQ in the septal membrane and establish how the proteins SpoIIQ anchors are organized around the forespore. 2) Determine how membrane-tethered cell wall hydrolases and associated proteins govern membrane migration around the spore during engulfment. 3) Define the mechanism by which membrane fusion is catalyzed. 4) Investigate how membrane protein localization and dynamics are influenced by membrane composition, membrane microdomains, and the MreB cytoskeleton.